1 Life-threatening infections associated with multi-drug resistant (MDR) healthcare acquired pathogens 2 (HAPs) leading to sepsis and multiple organ failure are the most deadly, disabling and costly complications facing 3 the critically ill and injured today. As advances in early injury care have improved, we are now witnessing a new 4 threat, late onset sepsis- today the most common cause of deaths following major surgery, trauma and burn 5 injury. While newer and more powerful antibiotics are certainly needed, deploying a broad ?kill strategy? with 6 antibiotics carries the unintended consequence of disruption of the intestinal microbiome and the further 7 emergence of antibiotic resistance. In this proposal we present compelling preliminary data to demonstrate that 8 catabolic injury itself results in a major alteration in intestinal microbiota composition and function which has a 9 direct and negative impact on local and systemic immunity. We show, for the first time, that provision of normal 10 microbiota via fecal microbiota transplant (FMT), delivered as an enema to mice intestinally infected with, and 11 septic from, a well characterized multi-drug resistant human pathogen community (PC), results in their complete 12 ?rescue? from what would otherwise be a fatal course of gut-derived sepsis. Whole genome transcriptome 13 analysis of host tissues (liver, spleen, cecum) demonstrated that the FMT induces a recovery-directed immune 14 response at the systemic level. Most strikingly, when these same lethal multi-drug resistant human pathogens 15 are introduced intraperitoneally (IP) and mice develop gross systemic sepsis, FMT rescues 100% of mice to 16 complete health. Whole genome sequencing and use of knockout mice provide key evidence that the protective 17 effect of FMT in both models is mediated by type I interferon signaling at the systemic level. Therefore in this 18 proposal we hypothesize that the normal intestinal microbiota play a key and underappreciated role in 19 the outcome from critical illness and infection via their action on immune cells that enhance pathogen 20 clearance mechanisms. In this proposal we will elucidate the key role that the intestinal microbiome plays on 21 immune clearance mechanism by 1. determining the regional distribution and degree to which, a fecal microbiota 22 transplant (FMT), delivered as an enema, repopulates the composition and function of the intestinal microbiome 23 in septic mice with multi-pathogen peritonitis, 2. determining the mechanisms by which an FMT rescues septic 24 mice from multi-pathogen bacterial peritonitis via its activation and enhancement of peritoneal macrophage 25 phagocytic function, and 3. elucidating the mechanisms by which the intestinal microbiota suppress the 26 emergence of antibiotic resistant and lethal bacterial phenotypes following major surgical injury. Results of these 27 proposed studies has the potential to transform the way we manage critically ill and injured patients by developing 28 strategies (i.e., FMT) that maintain the composition and function of the intestinal microbiota to a degree sufficient 29 to enhance immune function and decrease the rate of multi-drug resistant infections, late onset sepsis and overall 30 mortality.